This invention relates generally to the field of cryogenic storage devices and, more particularly, to an improved cryogenic dewar having a tray for holding specimens, the tray including a thermally conductive, cylindrical sleeve, containing a skirt which is at least partially immersed in liquid cryogen.
Vapor phase liquid cryogen freezers have been used for several decades for long term storage of biological specimens, which are heat sensitive. Normally, a frozen specimen is placed into a storage container, which is stored in a dewar. A typical dewar 10, shown in FIG. 1, contains an outer shell 12 housing inner tank 14, separated from inner shell 16 by vacuum-insulated space 18. Inner tank 14 is closed using lid 34.
A stainless steel turn tray 30 holds a number of stainless steel storage racks 32 with shelves 33, where vials of biological specimens are placed in boxes on the shelves 33 for storage. The racks 32 rest on a circular, stainless steel, turn tray platform 26 welded to the remainder of the tray. Vertical dividers 24 separate turn tray 30 into sections, each of which may hold one or more racks 32. For example, four dividers may be used to separate tray 30 into quadrants.
A cylindrical sleeve 36, made of stainless steel and welded to the edges of dividers 24, surrounds tray 30. Sleeve 36 and dividers 24 cooperate to help maintain the storage racks 32 placed between dividers 24 in an upright position by keeping the racks 32 from tipping over within their particular sections. The sleeve 36 of the prior art dewar extends upwardly from platform 26 to the top of vertical dividers 24.
Dividers 24 and platform 26 are welded to a stainless steel central tube 28 to allow tray 30 to rotate within inner tank 14. To access storage racks 32, a user rotates tray 30 using handles 20 attached to the top edge of dividers 24, until a desired rack 32 is positioned underneath lid 34, whereby a desired specimen may be acquired by removal of the rack 32.
The bottom of the inner tank is a reservoir for a pool of liquid cryogen 40, such as liquid nitrogen. As the nitrogen receives heat transferred from outside of dewar 10, via inner shell 16 and lid 34, a portion of the nitrogen evaporates to produce a cold vapor, which surrounds the storage racks 32. This type of cold storage, known as xe2x80x9cvapor phasexe2x80x9d storage, prevents cross-contamination of the biological specimens stored within dewar 10. The nitrogen vapor passes through apertures 25 within dividers 24 and platform 26.
A primary concern of such vapor phase storage is maintaining a desired, low temperature at the storage racks, particularly at the upper shelves. While liquid nitrogen at the bottom of the dewar remains at a constant temperature (about xe2x88x92196xc2x0 C.), and while vapor near the liquid nitrogen approaches this temperature, ambient heat entering from the walls and lid of the container warm the vapor above the liquid pool. This warmer vapor migrates to the upper portions of inner tank 14, and thus to the specimens contained on the upper shelves. A temperature gradient of as much as 100xc2x0 C. can exist from the bottom of the dewar to the top. This difference is significant, because it is accepted that diffusion within biological specimens can begin to occur at temperatures as warm as xe2x88x92132xc2x0 C. Keeping the temperature of the specimens under this threshold is thus a significant concern. Storage below xe2x88x92150xc2x0 C. is generally accepted by the industry as safe since it is below the threshold for diffusion by a safe margin to allow for temperature fluctuation in the freezer.
Past efforts to decrease the temperature gradient, and thereby lower the upper shelf temperatures, fall into two categories. The first is improving the insulation efficiency of the dewars, which indeed lowers the temperature gradient for a closed dewar. However, once the lid of the dewar is opened, heat enters the dewar, adversely affecting, the top shelves. The top shelves can get quite warm (about xe2x88x9250xc2x0 C.), and there is a slow recovery time for the shelves to revert to a cooler temperature.
A second solution is making the shelving and rack out of aluminum or a similar metal with high thermal conductivity. While at steady state temperatures, with the lid closed, this method appears to solve the problem, but it is actually worsened when the dewar lid is opened to add or remove samples. As heat enters the dewar through the open lid, the aluminum shelving and rack transfer significant heat to the lower shelves. This is because the nitrogen vapor is a poor thermal conductor and doesn""t effectively transfer the heat to the liquid nitrogen pool below.
Accordingly, an improved cryogenic freezer, which has a lower temperature gradient from the bottom to the top, and that can keep the top shelves at a relatively constant temperature, below a desired threshold, is needed.
It is an object of the invention to provide a liquid cryogen freezer that prevents specimens stored within the freezer from exceeding a desired threshold temperature.
It is a further object of the invention to provide a dewar that significantly lowers the temperature gradient.
It is a further object of the invention to provide a liquid cryogen freezer that allows a user to quickly and easily access desired biological specimens within the freezer, while maintaining a safe temperature for the specimens.
It is a further object of the invention to provide a liquid cryogen freezer that reduces the time of the dewar to recover to steady state storage conditions after the lid has been opened.
It is a further object of the invention to provide a liquid cryogen freezer that maintains a nearly uniform steady state temperature within the dewar, even when the lid of the dewar is opened.
The present invention overcomes the shortcomings of the prior art, and consists of a dewar with an improved turn tray having a sleeve made of a thermally conductive material. The sleeve of the improved tray has a thermally conductive skirt extension, which extends below the floor of the tray so as to be at least partially immersed in the pool of liquid nitrogen contained in the inner tank. The sleeve extends upwardly to a level substantially even with the top of the storage racks.
The sleeve is in direct contact with the liquid nitrogen and is an excellent thermal conductor. Heat entering the tank through the lid is rapidly transferred into the liquid nitrogen pool below via the sleeve instead of into the nitrogen vapor surrounding the stored specimens. This, in turn, increases evaporation of the liquid nitrogen producing additional cool vapor that reaches the top storage shelves more quickly than in prior art dewars thereby decreasing the time required for the dewar to recover to steady state conditions. As a result, the temperature gradient is significantly decreased and the upper storage shelves are maintained at a safe temperature.